High-reliability processing systems often require duplication of the processor (including processor memory), so that it is not a single-point-of-failure in the system. Usually, one processor is active and runs the system, and the other processor is kept in "standby" mode, where it is continually updated with copies of memory-write transactions occurring in the active processor (this updating process is called "shadowing"). If the active processor has a fault, then the system switches to the standby processor and continues the system's processing tasks virtually glitchlessly, since it has knowledge of the state of the active processor at the time of the failure.
A commonly-used shadowing method copies all data from the active processor to the standby processor's memory, even though only a portion of it is actually needed in order for the standby processor to continue operating the system in case of an active processor fault. This method of shadowing has become a serious bottleneck for high-performance processing systems. It reduces overall system performance, as it usually takes longer to transfer data from the active processor to the standby processor than it takes to perform a normal memory-write operation on the active processor.
Another known method of shadowing is called "transaction-based duplication" or "software duplication". The basic idea is that the active processor sends information about what functions it is performing to the standby processor, and the standby processor duplicates these functions and in the process updates its memory state to that of the active processor. While it has very little hardware impact, other than the need for a transaction channel from the active to the standby processor, the complexity of this method lies in the software. The reason for the complexity is that all of the applications code must be able to perform, and must actively manage, the duplication function.
What the art lacks is a shadowing method that would allow either for not shadowing some portions of memory, and/or for dynamic control of which portions of memory are shadowed and which are not.